1. Field of the Invention
The present invention relates to a rotation anode X-ray tube which is mounted on an X-ray image diagnostic system, a nondestructive inspection system, or the like.
2. Description of the Related Art
A rotation anode X-ray tube which is mounted on an X-ray image diagnostic device, a nondestructive inspection system, or the like, and which is used as a source of release of X-rays has been known. This rotation anode X-ray tube has an anode target which generates X-rays by electron collision, an electron emitting source which emits electrons toward the anode target, and a vacuum envelope which keeps at least the periphery of the anode target and the electron emitting source at a predetermined degree of vacuum.
The electrons emitted from the electron emitting source are accelerated by a voltage applied between the anode target and the electron emitting source, and are made to collide against a focal plane of the anode target. The electrons which have collided against the anode target are converted into heat and X-rays on the anode target, and some of generated X-rays are outputted from an X-ray transmission window provided at the vacuum envelope.
However, among the electrons which have collided against the anode target, there are some electrons which have not been converted into heat or X-rays, but become recoil electrons to repeatedly scatter about. A direction and intensity of recoil electrons are changed in accordance with an applied voltage or an electric field in the vicinity of a focal point. However, usually, 40% or more of incident electrons recoil in all directions.
Recoil electrons return to portions other than the focal plane of the anode target, or rush into the vacuum envelop. Heat and X-rays are generated due to the recoil electrons returning or rushing-in.
X-rays generated by recoil electrons become a noise component with respect to X-rays generated from the focal plane of the anode target, which is impeditive for obtaining uniform X-rays. Further, heat generated by recoil electrons causes a rise in temperature of the anode target or the like.
Then, in order to solve these problems, there has been proposed a rotation anode X-ray tube in which recoil electrons returning to an anode target and recoil electrons rushing into a vacuum envelop are reduced by capturing generated recoil electrons. This rotation anode X-ray tube has a recoil electron capturing structure functioning as a trap for capturing recoil electrons between the anode target and the electron emitting source.
FIG. 6 is a partially cutaway perspective view showing a recoil electron capturing structure 100 in a conventional art. As shown in FIG. 6, the recoil electron capturing structure 100 is formed in a cylindrical shape so as to surround an orbit of electrons e heading from the electron emitting source toward the anode target, and captures recoil electrons re which have recoiled on the anode target by utilizing the inner peripheral surface thereof. In addition, a flow channel 101 for allowing coolant to flow is formed along the circumferential direction inside the peripheral wall of the recoil electron capturing structure 100, and heat generated by capturing recoil electrons is let out to the outside by the coolant flowing in the flow channel 101 (for example, in Jpn. Pat. Appln. KOKAI Publication No. 2002-352756 (on the third to fifth pages, FIG. 1).
Generally, electrons having extremely high energy are thrown into the anode target. Therefore, heat generation of the recoil electron capturing structure is made enormous, which requires intensive cooling. In accordance therewith, a great temperature gradient is brought about between a heating unit and a cooling unit of the recoil electron capturing structure, and as a result, a great thermal stress is generated at the junction between the recoil electron capturing structure and the vacuum envelop.
Generally, in many cases, the recoil electron capturing structure is structured based on a copper material having high thermal conductivity in order to let enormous amount of generated heat out to the outside as soon as possible. In particular, pure copper is excellent at thermal conductivity and brazing flowability, and is relatively inexpensive, and therefore, it is used in many cases.
However, pure copper easily brings about secondary recrystallization which is called surface roughness by repeating thermal stress as described above. When secondary recrystallization proceeds, generation of gas from crystalline boundaries, reduction in surface roughness, and the like are brought about by boundary sliding or the like, which results in deterioration in withstand voltage. Namely, there is a defect that the life span is short in a recoil electron capturing structure formed from pure copper as a material.
Then, in recent years, in order to improve the short life span of pure copper, oxide-dispersion-strengthened copper whose mechanical strength is enhanced by dispersing oxide in pure copper has been used. One example thereof is alumina (aluminum oxide) dispersed copper and the like. Further, strengthened copper alloy whose mechanical strength has been enhanced by making a copper alloy by mixing a slight amount of dissimilar metal into pure copper has also been used. One example thereof is copper alloy such as chrome, tungsten, and the like.
Both of oxide-dispersion-strengthened copper and strengthened copper alloy are used for the purpose of enhancing the mechanical strength while keeping the high thermal conductivity of copper to some extent, and the defect in pure copper described above can be improved to some extent by using those as materials.
However, because oxide-dispersion-strengthened copper and strengthened copper alloy have ductility lower than that of pure copper, when crystal breaking is once brought about, the breaking becomes cracks, which rapidly proceed and finally lead to atmospheric penetration in some cases. Namely, there is a defect that it is impossible to keep vacuum tight at the inside of the vacuum envelop in a recoil electron capturing structure formed from oxide-dispersion-strengthened copper or strengthened copper alloy as a material.
Next, proceeding of cracks and the effect thereof in a recoil electron capturing structure formed from alumina-dispersed copper as a material will be described in detail with reference to FIGS. 7 and 8.
FIG. 7 is a plan view of a recoil electron capturing structure by using alumina-dispersed copper as a material in the conventional art, and FIG. 8 is a cross-sectional view of the recoil electron capturing structure by using alumina-dispersed copper as a material in the conventional art.
Cracks C generated on the inner peripheral surface of the recoil electron capturing structure 100 proceed along radial directions of the recoil electron capturing structure 100, and penetrate up to the flow channel 101 formed inside the recoil electron capturing structure 100 as shown in FIGS. 7 and 8. Note that, because the flow channel 101 is connected to a cooler installed at the outside of the vacuum envelop, the fact that the cracks C penetrate up to the flow channel 101 means that the cracks C bring about atmospheric penetration.
In particular, with respect to oxide-dispersion-strengthened copper such as alumina-dispersed copper and the like, a drawing process or an extrusion process is used as a method for manufacturing the material. Therefore, in many cases, a specific crystal orientation is brought about in the material in consequence of the drawing process or the extrusion process. Further, there is a trend that a great force to be enlarged radially by heating is applied to the recoil electron capturing structure. Accordingly, when a crystal orientation of the oxide-dispersion-strengthened copper and an axial direction of the recoil electron capturing structure are matched with each other, a force is applied to the recoil electron capturing structure so as to pull away crystal fibers from each other, which makes generated cracks easily progress in radial directions of the recoil electron capturing structure 100.
Moreover, when a recoil electron capturing structure formed from an oxide-dispersed copper, a strengthened copper alloy, or the like as a material is used for a rotation anode X-ray tube, as long as generated cracks are small, withstand voltage is hardly affected. Therefore, in some cases, the rotation anode X-ray tube is made unusable at a point in time when atmospheric penetration is brought about finally due to cracks proceeding insidiously. Namely, there is a possibility that the rotation anode X-ray tube becomes suddenly unusable, which is unfavorable for medical use.
Further, in many cases, the recoil electron capturing structure is joined with a vacuum envelop 102 by brazing with copper serving as a brazing filler metal. However, when an oxide-dispersed copper, a strengthened copper alloy, or the like is used as a material of the recoil electron capturing structure, there is a defect that the brazing flowability with respect to the recoil electron capturing structure is deteriorated, and stress peeling and the like are easily brought about at the junction between the recoil electron capturing structure and the vacuum envelop 102.
To summarize the description, because an enormous amount of heat is generated in a recoil electron capturing structure, a copper material having high thermal conductivity, and a structure of internal forced liquid-cooling are used. However, when pure copper is used as a material for a recoil electron capturing structure, gas emission due to surface roughness and short life span due to deterioration in withstand voltage are brought about by repeating thermal stress during use. On the other hand, because cracks easily proceed in oxide-dispersion-strengthened copper and strengthened copper alloy which are used for elongating life span as long as possible, when an oxide-dispersion-strengthened copper or a strengthened copper alloy is used as a material for a recoil electron capturing structure, there is the risk that penetration-leakage defect is suddenly brought about.